Thin, planar, durable, easily manufacturable and relatively large area light sources having a range of light intensities are useful in many applications. Such light sources may be useful in backlights for LCDs to improve readability in all ambient lighting situations. They are also commonly used in night vision and avionics applications and, if filed (i.e., several lamps are positioned adjacently in a two-dimensional matrix), may be useful in sign applications to provide a uniform light source for illuminating a graphic image.
In some applications incandescent lights or LED arrays can be used to form planar light sources. However, these devices typically face the limitations of lack of uniformity of light, high power consumption, and generation of undesirable heat.
An alternative often chosen in modem applications is fluorescent technology. Tubular fluorescent lamps have the advantage of being relatively efficient, generating relatively bright light, and having well-established manufacturing capability. Tubular fluorescent lamps suffer, however, from their fragility, their requirement for optical elements to reflect and diffuse light to provide a uniform display, and limited capability to operate efficiently and effectively in low light applications.
A more desirable technology in many applications is the planar fluorescent lamp. Planar fluorescent lamps are known in the art, having been described, for example, in U.S. Pat. Nos. 3,508,103; 3,646,383; and 3,047,763. Typically, such lamps in the prior art are formed by molding a housing and a cover, each from a piece of glass and sealing the glass pieces to form a sealed enclosure. A selected gas and a fluorescent material are placed in the sealed enclosure for emitting light when an electrical field is applied.
Where the enclosure is formed entirely from glass, fabrication can be difficult and the resulting lamp is often quite fragile. A stronger lamp can be made by using thicker pieces of glass to form a lamp having thicker walls. However, increased glass thickness results in extra weight, is more difficult to fabricate and may attenuate some light output. Further, all forming and annealing is done with heat processing equipment, which is expensive and requires special handling of materials due to the high temperature of processing. Additionally, because such processing requires controlled temperatures during cooling to prevent defects caused by cooling, the process is quite lengthy. These lamps also typically result in operation at higher temperatures than is desirable.
Planar fluorescent lamps having sidewalls formed from metal with a serpentine channel defined by separate strips are known from U.S. Pat. Nos. 3,508,103 and 2,405,518. These lamps require fabrication and assembly of several elements to form the lamp body. Further, after such lamps are assembled and the glass cover is attached, the glass cover is typically not sealed to the tops of the metal strips defining the channels. Consequently, small gaps may remain between adjacent channels which can reduce the overall discharge length of the lamp by permitting the discharge to "shortcut" between adjacent sections of the serpentine channel, rather than following the defined serpentine channel. As is known in the art, a reduced discharge length reduces the overall efficiency of the lamp. Additionally, such an effect causes darkening of those sections of the channel through which the discharge does not travel, thereby reducing the overall uniformity of the lamp. Such a shortcut of the discharge may also cause localized heating which may in turn damage the lamp.
An alternative approach disclosed in U.S. Pat. No. 4,767,965 describes a lamp formed from two parallel glass plates supported by a frame piece. The '965 patent describes a lamp that employs two cold cathode electrodes placed opposite each other. Because the plasma discharge at an optimum mercury vapor pressure conducts current as an arc, it generates light non-uniformly in such a lamp. While the cold cathode electrodes may simplify construction, the lamp described in this patent suffers from brightness variations as great as 60% across the face of the lamp. Additionally, the glass plates used in the lamp must be thick to withstand atmospheric pressure when the enclosure is evacuated.
A need remains, therefore, for a thin, planar lamp having a substantially uniform display which is easily manufacturable, provides a sealed serpentine channel, has a relatively broad range of light intensifies, is temperature tolerant, and is relatively durable. Also, such a lamp, preferably would provide illumination from out to its periphery, allowing multiple lamps to be tiled.